Improvements in prosthetic intervertebral disc and joint replacement components, and related surgical procedures, have led to dramatic increases in implant longevity. Many artificial hip and knee components now last for twenty years or more due to improved materials and greater insight into movement, load distribution and wear characteristics.
Many spinal conditions, including degenerative disc disease, can now be treated through artificial disc replacement (ADR), which has several advantages over spinal fusion. The most important advantage of ADR is the preservation of spinal motion. Spinal fusion eliminates motion across the fused segments of the spine. Consequently, the discs adjacent to the fused level are subjected to increased stress, which increases the changes of future surgery to treat the degeneration of the discs adjacent to the fusion.
One of the most important features of an artificial disc replacement (ADR) is its ability to replicate the kinematics of a natural disc. ADRs that replicate the kinematics of a normal disc are less likely to transfer additional forces above and below the replaced disc. In addition, ADRs with natural kinematics are less likely to stress the facet joints and the annulus fibrosus (AF) at the level of the disc replacement. Replicating the movements of the natural disc also decreases the risk of separation of the ADR from the vertebrae above and below the ADR.
In an attempt to replicate natural disc movements various ADR materials have been tried, including hydrogels, metal and rubber. As one example, U.S. Pat. No. 6,602,291 resides in a prosthetic spinal disc nucleus comprising a hydrogel core surrounded by a constraining jacket. The hydrogel core is configured to expand from a dehydrated state to a hydrated state. In the dehydrated state, the hydrogel core has a shape selected to facilitate implantation through an annulus opening. Further, in the hydrated state, the hydrogel core has a shape corresponding generally to a portion of a nucleus cavity, the hydrated shape being different from the dehydrated shape. Upon hydration, the hydrogel core transitions from the dehydrated shape to the hydrated shape.
Unfortunately, the flexibility of the hydrogel and the constraining jacket allow hydrogel ADRs to change shape and extrude through defects in the annulus through which the ADR was inserted, for example. My U.S. Pat. Nos. 6,245,107, 6,371,990, 6,454,804, and published applications WO 01/10316 A1; 20020156533; 20020165542; 20030004574; 20030040796; and 20030078579 are useful in addressing such problems.
Metal and rubber ADRs, on the other hand, also frequently fail at the metal-rubber interface. The rubber fails directly due to high shear stresses or because the rubber separates from the metal. Clearly any improvements in these and other areas would be welcomed by the medical community and by patients undergoing procedures to implant prosthetic components of this kind.